The invention relates to a prism system for a variable binocular tube comprising a beam splitting prism, an eyepiece prism having one effective plane of reflection and an eyepiece prism having two effective planes of reflection.
The beam splitting prism has reflecting and light transmitting faces and one beam splitting plane which splits incident object beams into a reflected and transmitted portion each and fuses the latter to two combined object beams.
The prism system can find application in diverse optical devices such as microscopes, telescopes etc. and can replace monocular observation units.
Previous prism systems for binocular tubes comprise a cube prism centrally arranged including a diametrically arranged beam splitting face which splits the incident beams into two initially diverging eyepiece beams, (refer also to Brockhaus "ABC der Optik" VEB. F. A. Brockhaus press, Leipzig, 1969, page 575).
Two subsequent eyepiece prisms render the two beams into parallel ones, the one partial beam being once reflected and the other partial beam being twice reflected subsequent to the beam splitting face.
The entire deflection by the system relative to the beam splitting face is towards the twice reflecting eyepiece prism. It is a particular feature of the previous prism systems that the one eyepiece prism only effects a lateral displacement of the beam, whereas the other only once reflecting eyepiece prism deflects the beam by 90.degree..
Furthermore, the optical path lengths of the light in glass and in air are substantial equal in both paths of beam and the light entrance faces and the light exit faces of all prisms are at right angles to the optical axis of the respective passing path of beam.
Two technical solution are known for adjustment of the eyepiece space at a constant optical path length between the tube attachment and the field of view aperture of the eyepieces. In the one solution the tube sockets for receiving the eyepieces are tiltable about a common axis about like angles.
It is, however, disadvantageous that the variation of the space between the two eyepieces obtained also involves a variation of the orientation of the reticle in the intermediate image plane of the eyepieces.
In the other more suitable technical solution the tube sockets for receiving the eyepieces are diametrically adjustable. The means for guiding the two base plates to set the eyes' space and guidings mounted on each base plate for simultaneously effecting a space variation between the eyepiece prism and the tube sockets are arranged at right angles to each other and in parallel to the path of beams of the eyepieces. Only the last, considered in direction of light propagation, of the operative reflection faces is displaced in both eyepiece path of beams so that tilting errors may occur on both sides.
The optical system of the binocular tubes is constituted of cubic and semi-cubic prisms and a semi-transparent beam splitting face.
The introduction of a second non-displaceable path of beams is only feasible with the diametrically displaceable binocular tube type.
The introduction place is, however, poorly accessible since it is located directly under the beam displacing eyepiece prism.
The two tubes and the coupling faces include an angle of 90.degree. at a like optical path length. Since there are no common coupling faces, misadjustments easily result.
It is no advantage to exchange the two eyepieces in order to use the different number of reflections at an unchanged optical path length since this involves an unfavorable position for the operator.
A further disadvantage is involved through the comparatively wide angle of incidence of 45.degree. relative to the beam splitting face.